Transmitting soliton pulses or solitons in the part of an optical fiber with abnormal dispersion is known in itself. Solitons are pulse signals with a sech.sup.2 pulse shape. With pulses of this shape, the non-linearity in the corresponding part of the fiber compensates the dispersion of the optical signal. Modeling transmission of solitons using the non-linear Schrodinger equation is known in itself.
Various effects limit the transmission of such pulses, such as the jitter induced by interaction of the solitons with noise present in the transmission system, as described for example in the article by J. P. Gordon and H. A. Haus, Optical Letters, Vol. 11, No. 10, pages 665-667. This effect, known as the Gordon-Haus effect or Gordon-Haus jitter, imposes a theoretical limit on the quality or bit rate of soliton transmission. Synchronous modulation of the soliton signals using semiconductor modulators is one way of exceeding this limit. Systems of sliding guide filters for controlling the jitter of the transmitted solitons have also been proposed, for example in EP-A-0 576 208. Using the Kerr effect in synchronous amplitude or phase modulators and using saturable absorbers to regenerate the signal on the line have also been proposed.
Using wavelength-division multiplexing (WDM) to increase the bit rate of soliton signal fiber optic transmission systems has also been proposed.
In this case, asymmetric collisions between the solitons of the various channels cause unwanted time-jitter. This jitter is inherent to the use of lumped amplifiers: the solitons of the various channels of the multiplex do not interact in the same way at low power (before amplification) and at high power (after amplification). This time-jitter exists both in passive systems, such as guide filters, and in active systems, for example systems using synchronous regeneration.
This problem is discussed in A. F. Evan and J. V. Wright, "Constraints on the design of single channel high capacity (&gt;10 Gbit/s) soliton systems", IEEE Photon Technology Letters, Vol. 7, No. 1, p. 117 (119); this article explains that the problem of Gordon-Haus jitter is the limiting factor for 10 Gbit/s transoceanic systems using solitons; however, for transmission at higher bit rates, the transmission distance or the distance between amplifiers is limited by interference caused by lumped amplification. That article suggests using fiber with decreasing dispersion or distributed optical amplification, which could locally compensate the dispersion and non-linearities of the fiber.
A first type of solution that has been proposed consists in correcting the losses in the fiber by appropriately controlling the dispersion. D. J. Richardson et al., "Periodically amplified system based on loss compensating dispersion decreasing fiber", Electronics Letters, Vol. 32, No. 4, p. 373 (1996) proposes an experimental configuration of a recirculating loop using line fibers whose dispersion profile follows the loss profile. Using line fiber whose dispersion profile decreases exponentially between amplifiers enables transmission distances in the order of 266 km to be achieved in this system; the system is nevertheless limited by time-jitter.
An article by L. F. Mollenauer et al., "Wavelength Division Multiplexing with Solitons in Ultra Long Distance Transmissions using Lumped Amplifiers", Journal of Lightwave Technology, Vol. 9, No. 3, pages 362-367 (1991) describes the problem of collisions between solitons in wavelength-division multiplex transmission systems and emphasizes the variations in propagation speed induced by collisions; that article explains that variations in the chromatic dispersion of the fiber along the transmission path can compensate the effects of collisions; it therefore proposes using segments with different dispersion to compensate the effects of collisions on the soliton propagation speed.
An article by A. Hasegawa, S. Kumar and Y. Kodoma, "Reduction of Collision-Induced time-jitter in Dispersion-managed Solitons Transmission Systems", Optics Letters, Vol. 21, No. 1, January 1996, pages 39-41, proposes a fiber dispersion management scheme which increases the distance between amplifiers. That solution is based on a stepped dispersion profile in the fiber approximating an ideal exponential profile as closely as possible.
The above solutions remain difficult to implement on an industrial scale: it is necessary to produce large quantities of fiber having a specific dispersion profile, to concatenate 20 km segments and to splice the fiber to other components. From the production point of view, the solution consisting in approximating the exponential dispersion profile by steps of constant dispersion is subject to the same control and segment assembly constrains; moreover, the acceptable tolerances for the dispersion and the segment length decrease rapidly as the number of steps decreases.
Another type of solution consists in using distributed amplification along the fiber.
L. F. Mollenauer et al., "Soliton propagation in long fibers with periodically compensated loss", IEEE Journal of Quantum Electronics, Vol. QE-22, No. 1, p. 157 (1986) describes digital simulation of a transmission system in which losses are periodically compensated by the Raman gain. For this purpose, the article proposes periodically (every 50 km) injecting a pump into the line fiber, the difference between the wavelength of the solitons and the wavelength of the pump being chosen to correspond to the peak Raman gain in the fiber. The article also considers the effect of the collision between two solitons of a wavelength-division multiplex to show the theoretical possibility of wavelength-division multiplex transmission, despite the effects of collisions on the solitons.
A. Hagesawa, "Amplification and reshaping of optical solitons in a glass fiber--IV: use of the stimulated Raman process", Optics Letters, vol. 8, No. 12, p. 650 (1983) does not describe the problem of the asymmetry of collisions between solitons of adjoining channels; that article proposes a single-channel transmission system using distributed amplification by the stimulated Raman effect; in the example given, powers in the order of 70 mW are injected every 43 km.
J. R. Simpson et al., "A distributed Erbium doped fiber amplifier", Proc. Conference on Optical Communications, OFC'90, paper PD19 (1990) does not describe the problem either, but proposes a method of fabricating a fiber for an erbium-doped fiber distributed amplifier; the erbium concentrations obtained are sufficiently low to enable distributed amplification along the line fiber.
Similarly, S. T. Davey et al., "Lossless transmission over 10 km of erbium-doped fiber using only 15 mW pump power", Electronics Letters, Vol. 26, No. 2, p. 1148 (1990) proposes another method of fabricating a fiber for an erbium-doped fiber distributed amplifier. That article mentions application to soliton systems, but not the problem of the asymmetry of collisions.
That type of distributed amplification poses problems; high pump powers are needed to obtain the stimulated Raman effect; these powers are typically in the order of 300 mW at wavelengths of 1.45 mm for 30 km segments with pumping in both directions. Although such powers are technically feasible, the necessary power and reliability in practice rule out this solution. Erbium-doped fiber distributed amplification systems have a high noise level compared to lumped amplifiers: noise levels are typically 7 to 9 dB for a 50 km segment, compared to values in the order of 4.5 dB for a segment of the same length using lumped amplification; the pump power needed to assure the transparency of the link is also fairly high, typically 100 mW, or to be more precise 50 mW in each direction, at 1.48 mm.
Finally, it has also been proposed to use very closely spaced amplifiers. The aforementioned article by L. F. Mollenauer et al. in the Journal of Lightwave Technology also shows that solitons at different wavelengths are transparent to each other in a lumped amplification transmission system if the length of the collision, i.e. the distance that the solitons travel along he fiber while they are traversing each other, is large relative to the spacing of the amplifiers. The article proposes using lumped amplifiers with a spacing of 20 km. This solution is difficult to implement for economic reasons.
On the subject of wavelength-division multiplex transmission, an article by E. Desurvire, O. Leclerc, and O. Audouin, Optics Letters, Vol. 21, No. 14, pages 1026-1028 describes a wavelength allocation scheme which is compatible with the use of synchronous modulators. The article proposes to allocate wavelengths to the various channels of the multiplex so that, for given intervals Z.sub.R between repeaters, the signals of the various channels, or to be more precise the bit times of the various channels of the multiplex, are substantially synchronized when they reach the repeaters. That enables in-line synchronous modulation of all the channels at given intervals using lumped synchronous modulators. That multiplex wavelength allocation technique is also described in French Patent Application 96 00732 of Jan. 23, 1996 in the name of Alcatel Submarine Networks. In the article, it is proposed to choose a subgroup of channels which are synchronous not only at intervals Z.sub.R but also at intervals which are sub-multiples of Z.sub.R. Other aspects of this wavelength allocation technique are described in the articles by O. Leclerc, E. Desurvire, and O. Audouin, "Synchronous WDM Soliton Regeneration: Toward 80-160 Gbit/s Transoceanic Systems", Optical Fiber Technology, 3, pages 97-116 (1997) and by E. Desurvire et al., "Transoceanic Regenerated Soliton Systems: Designs for over 100 Gbit/s Capacities", Suboptic '97, pages 438-447.
It should be noted that the solutions to the problem of jitter due to asymmetric collisions proposed in the aforementioned articles are not necessarily compatible with this type of wavelength allocation scheme; controlling the dispersion of the fiber would interfere with the synchronicity of the bit times at the synchronous regenerators.
In the case of a single-channel soliton signal transmission system, a large amplification step, typically greater than Z.sub.0 /5, causes the generation of continuum or dispersive waves; in this case Z.sub.0 is the wavelength of the solitons.